CORRELATION RECEIVER WITH RAPID FREQUENCY, TIME AND PHASE ACQUISITION

• US 3,864,635 A
• Filed: 08/04/1972
• Issued: 02/04/1975
• Est. Priority Date: 08/04/1972
• Status: Expired due to Term

• Current Assignee: Walter Ewanus
• Original Assignee: Walter Ewanus
• Inventors: Ewanus, Walter
• Primary Examiner(s): Britton, Howard W.
• Assistant Examiner(s): Psitos, Aristotelis M.
• Application Number:
  US05/277,938
• Time in Patent Office: 914 Days
• Field of Search: 325/345,346,416,418-420,349,432,433,421,58
• US Class Current: 455/260
• CPC Class Codes: H04L 7/043 Pseudo-noise [PN] codes variable during transmission synchronisation of spread spectrum receivers H04B1/69

Abstract

A correlation receiver provides rapid time, frequency, and phase acquisition in accordance with an optimum search procedure utilizing both parallel and sequential search techniques. Time and frequency acquisition are performed simultaneously, providing both correlation of a replica code at the receiver with the pseudorandom sequence code modulation of the carrier for time acquisition and acquisition of the frequency within the predetermined bandwidth of a filter of a filter bank affording a parallel frequency search. A search lock loop provides phase acquisition and simultaneously controls a frequency tracking loop to acquire the precise frequency. Maintenance of code, and thus time, tracking and indication of correlation are achieved by introducing error signals in corresponding control loops, the former serving to drive the system to precise acquisition in all domains. A coherent output, as well as AM and FM outputs, are readily provided.

Claims

1. A recEiver for resolving frequency and phase uncertainties of a received signal, comprising means for identifying the frequency of a received signal within one of a plurality of differing slots each of limited frequency uncertainty and the plurality of differing frequency slots encompassing the entire range of frequency uncertainty of a received signal, means for generating a synthesized frequency signal having a first nominal frequency means for generating an adjustable frequency oscillation signal having a second nominal frequency so related to the first nominal frequency of the synthesized frequency signal that the combination of the first and second nominal frequencies corresponds to a third nominal frequency differing from the frequency of the received signal by a desired IF value, said synthesized frequency generating means being responsive to said frequency identification means to generate said synthesized frequency signal at a frequency value, relative to said first nominal frequency, which corresponds to the frequency slot identification of said received signal. means for mixing the adjustable frequency oscillation signal and the synthesized frequency signal to produce a combined signal having said third nominal frequency, means for mixing the combined signal with the received signal to convert the received signal to an IF signal having a nominal frequency of the desired IF value, means for producing a reference oscillation signal at the said desired IF value, phase detection means responsive to the IF signal and the reference oscillation signal to detect and produce an output representing any difference in phase therebetween and said adjustable frequency oscillation signal generating means is responsive to the output of said phase detection means to adjust the frequency of said adjustable frequency oscillation signal generated thereby, to achieve phase acquisition of the received signal

2. A receiver as recited in claim 1 wherein said means for generating a synthesized frequency signal comprises:

a frequency synthesizer having a store of predetermined, different frequency output signals corresponding to the differing frequency slots, logic means responsive to a frequency slot identification of a received signal by said frequency identifying means to identify the frequency of that slot, within the bandwidth of the slot, to said frequency synthesizer, and said frequency synthesizer responds to the output of said logic means to generate the respectively corresponding synthesized frequency signal

3. A receiver as recited in claim 1 wherein said frequency identifying means comprises:

a filter bank of plural filters of contiguous bandwidths respectively defining said differing frequency slots, the bandwidth of each filter establishing the frequency uncertainty of each frequency slot; and

means for supplying to said filter bank a received signal, an output from one of said filters of said bank thereby indicating receipt of a signal and its frequency within the frequency uncertainty of the bandwidth of the filter producing the output.

4. A receiver as recited in claim 3 wherein said supplying means of said frequency identifying means further comprises:

oscillation means producing a fixed frequency output, and further mixing means for mixing the received signal and the fixed frequency output of said oscillation means to supply said received signal to said filter bank at a reduced, desired center frequency relative to the total bandwidth of said filter bank, said fixed frequency of said oscillation means being selected to differ by the amount of said center frequency from the nominal frequency of the received signal.

5. A receiver as recited in claim 3 wherein said means for generating a synthesized frequency signal comprises:

a frequency synthesizer having a store of predetermined frequency output signals corresponding to the center frequencies of said filters, Logic means responsive to an output from one of said filters of said filter bank to identify the frequency of that filter for supply to said frequency synthesizer, and said frequency synthesizer responds to the output of said logic means to generate the respectively corresponding synthesized frequency signal.

6. A receiver as recited in claim 1 further including discriminator means receiving the IF signal for producing an FM output

7. A receiver as recited in claim 1 further comprising:

means for mixing the IF signal and the reference oscillation signal and producing an output representing the AM information content of the received signal, and filter means receiving said output of said mixing means for producing an AM output signal.

8. A receiver as recited in claim 1 which further comprises:

filter means receiving the output of said detector means for producing a coherent output.

9. A receiver as recited in claim 8 further comprising:

means for mixing the IF signal and said reference oscillation signal to produce a further combined signal, and means responsive to said further combined signal for producing an AM output.

10. A receiver as recited in claim 1 wherein said adjustable frequency reference oscillation signal generating means comprises:

a controlled frequency oscillator producing said second nominal frequency of oscillation, and a track loop filter responsive to the output of said phase detection detector means for controlling the output frequency of said controlled frequency oscillator to acquire and track the phase and thereby the frequency of the received signal.

11. A receiver as recited in claim 1 for resolving uncertainties in time of a received signal, produced by code modulation thereof in accordance with a known transmitter code and further comprising:

a code generator of an adjustable clocking rate for generating a replica of the transmitter code, means for mixing the replica code of said code generator with the received signal, and means for controlling the code generator to vary the clocking rate of the replica code produced thereby to attain correlation of the replica code with the code modulation of the received signal, said controlling means being responsive to a frequency identifying output from said frequency identifying means to terminate further variation of the clocking rate and to maintain the clocking rate of the code generator in synchronism with the code modulation of the received signal.

12. A receiver as recited in claim 11 wherein:

said controlling means modulates the clocking rate of the code generator at a first frequency to effect a phase modulation of the replica code and produce an amplitude modulation component of the received signal at a frequency corresponding to said first frequency, and there is further provided means for mixing the IF signal with the reference oscillation signal to produce an output a first filter tuned to said corresponding frequency of the resulting amplitude modulation and receiving said output of said means for mixing the IF signal and the reference oscillation signal, and producing an output in response to the modulation component thereof at said corresponding frequency, and said controlling means responds to said output from said first filter to control the modulation of the code generator clocking rate for maintaining correlation with the code modulation of the received signal.

13. A receiver as recited in claim 11 wherein:

said controlling means supplies and adds a digital signal of a second frequency to the replica code output of said replica code generator to produce an amplitude modulation component of the received signal of a known, corresponding frequency, and there is further provided means for mixing the IF signal with the reference oscillation signal to produce an output a second filter tuned to said known, corresponding frequency of the resultinG amplitude modulation, said second filter receiving said output of said means for mixing the IF signal and the reference oscillation signal, and producing an output in response to the resulting amplitude modulation component of said known, corresponding frequency to provide a correlation indication output.

14. A receiver as recited in claim 12 wherein:

said controlling means supplies and adds a digital signal of a second frequency to the replica code output of said replica code generator to produce an amplitude modulation component of the received signal of a known, corresponding frequency, and there is further provided. a second filter tuned to said known, corresponding frequency of the resulting amplitude modulation, said second filter receiving said output of said means for mixng the IF signal and the reference oscillation signal, and producing an output in response to the resulting amplitude modulation component of said known, corresponding frequency of the received and acquired signal to provide a correlation indication output.

15. A receiver for acquiring a received signal having phase and frequency uncertainties, comprising:

a first loop for performing a parallel frequency search of the received signal for frequency acquisition thereof within one of a predetermined number of frequency slots of differing center frequencies and acceptable bandwidths of frequency uncertainties, the totality of said frequency slots encompassing the entire bandwidth of frequency uncertainty of the received signal, and said first loop receiving the received signal and identifying and thereby acquiring the frequency thereof as within one of said frequency slots, and producing a synthesized frequency output corresponding in frequency to the thus acquired frequency of the received signal, a second loop for performing phase acquisition of the received signal, including means for producing a controlled frequency oscillation output means for mixing said synthesized frequency output of said first loop and said controlled frequency oscillation ouput of said second loop and producing a first mixed output, means for mixing said first mixed output with the received signal to convert said received signal to an IF signal, means for generating a reference oscillation signal at the frequency of the IF signal, and means for detecting the phase of the IF signal relative to said reference oscillation signal and producing an output corresponding to any detected phase difference therebetween, and said means for producing the controlled frequency oscillation is responsive to said output of said detection means to adjust the phase of the controlled frequency oscillation thereby to acquire the phase of the received signal.

16. A receiver as recited in claim 15 for acquiring a received signal having time uncertainty resulting from code modulation thereof in accordance with a known transmitter code, further comprising a time acquisition loop for achieving time acquisition simultaneously with said frequency acquisition of said first loop, including:

means for generating a replica code, means for mixing the replica code with said received signal, prior to supply of the received signal to said first and second loops, and means for altering the timing of said replica code to achieve correlation with the code modulation of the received signal, said altering means being responsive to the attainment of frequency acquisition by said first loop to terminate further altering of the replica code timing thereby to maintain correlation of the replica code with the code modulation of the received signal.

17. A receiver as recited in claim 16, further comprising a code tracking loop including:

means for modulating the clocking rate of the replica code at a given frequency to effect a phase modulation of the replica code and produce an amplitude modulation component of the received signal in said time acquisition loop at a known, coRresponding frequency displaced from the bandwidth of the information content of the received signal, means for deriving an amplitude modulation output from said second loop, and means receiving said amplitude modulation output of said second loop and responsive to the modulation component at said known, corresponding frequency for controlling said altering means to maintain said replica code in correlation with the code modulation of the received signal.

18. A receiver as recited in claim 16 further comprising a code tracking loop including:

means for adding to the replica code, a digital signal of a known frequency to produce an amplitude modulation component of the received signal, of a corresponding frequency displaced from the bandwidth of information content of the received signal, in said time acquisition loop, means for deriving an amplitude modulation output from said second loop, and means receiving said amplitude modulation output of said second loop and responsive to the amplitude modulation component at said known, corresponding frequency to provide a correlation indication.